Various attempts to fabricate practical and economical thermophotovoltaic (TPV) power generators have been reported over the years. However, generator designs based on the use of silicon photovoltaic cells have been unsuccessful because of a requirement for very high temperature emitters (T&gt;2300 K). Without very high temperature emitters, TPV systems based on silicon cells are both inefficient and operate at low power densities. Selective emitters based on rare earth oxides have been described (M. K. Goldstein, U.S. Pat. No. 4,976,606) which improve efficiencies but still suffer from low power densities at practical emitter temperatures. Low power density generators are not economical for large volume energy production.
In 1989, L. M. Fraas et. al. described a new GaSh photovoltaic cell sensitive in the infrared (IR) out to 1.8 microns. Later in 1989, M. D. Morgan, W. E. Horne, and A. C. Day proposed using GaSh cells in combination with a radioisotope thermal source for space electric power and in 1991, O. L. Doeliner proposed using GaSb cells looking at jet engine plumes to replace alternators on jet aircraft. Finally in 1992, A. P. Fraas and R. M. Fraas submitted U.S. patent application Ser. No. 07/906,452 describing a small, quiet, natural gas fired TPV generator using GaSb cells. Improvements on this natural gas fired TPV generator were subsequently described in a continuation-in-part U.S. patent application Ser. No. 08/047,477, filed Apr. 19, 1993.
These two applications describe a TPV generator in which multiple low bandgap photovoltaic cell strings are mounted around the perimeter of a cylinder parallel to a central cylindrical emitter, or IR radiator. The cell strings face radially inward to receive IR radiation from the emitter and efficiently convert this radiation into electric power. Shod pass IR filters are located between the emitter and the cells to pass the useful shorter wavelength IR on to the cells while reflecting the longer wavelength IR back to the emitter. A regenerator is located at the top of the burner/emitter so that the exhaust gases preheat the supply air, allowing for a higher flame temperature thereby making more short wavelength IR available to the photovoltaic cells.
The emitter in the above cylindrical TPV generator is designed to operate at between 1700 and 2000K and at this temperature, both the burner and emitter must be fabricated using ceramics, not metals. Although it is possible to design a ceramic emitter/burner with a uniform emitter temperature along its length by using small flame jets from a central burner tube with the jet pattern tailored for staged fuel addition as was described in our earlier applications, complex ceramic shapes are only available through special fabrication orders at high prices with long lead times. There is a need for a simple burner/emitter design for use with a cylindrical TPV generator, preferably one that uses off-the-shelf ceramic tubing. A second problem with the flame jet design is the need for substantial combustion gas supply pressure to force fuel and air through the small jet orifices. There is a need for a burner design with high conductance and a small pressure drop.